1. Field of the Invention
The present invention relates to a method of adjusting a reduction projection exposure device and, more particularly, to a method of a reduction projection exposure device which is designed to make it possible to reliably determine whether an illumination optical system should be adjusted or a reduction image-forming optical system should be adjusted when the reduction projection exposure device is adjusted to suppress a dimensional variation in transfer pattern in one exposure field.
2. Description of the Related Art
In the processes of manufacturing a semiconductor device, photolithography is popularly used in patterning of various portions such as a gate electrode, a wiring, a contact hole, and the like. The photolithography is constituted by the step of transferring a mask pattern of a photomask to a photoresist film on a wafer and the step of processing an underlying layer using the patterned photoresist film as a mask. In the process of transferring the pattern, the pattern is exposed by an exposure device through the photomask to transfer the mask pattern to the photoresist film.
The configuration of the reduction projection exposure device will be described below with reference to FIG. 1. FIG. 1 is a typical view showing the configuration of the reduction projection exposure device.
A general reduction projection exposure device 10, as shown in FIG. 1, includes a light source 12, an illumination optical system 14, and a reduction image-forming optical system 16.
The light source 12 comprises a mercury vapor lamp 18 serving an emission source and an elliptical mirror 20 for converging light emitted from the mercury vapor lamp 18.
The illumination optical system 14 comprises a mirror 22, a fly-eye lens 24, a mirror 26, and a condenser lens 28. The illumination optical system 14 causes light emitted from the light source 12 to be incident on the condenser lens 28 with a uniform light intensity distribution by the mirror 22, the fly-eye lens 24, and the mirror 26, and converges the light.
The reduction image-forming optical system 16 comprises a mask (reticle) 30 having a pattern to be exposed and a reduction lens 32. The reduction image-forming optical system 16 irradiates the light converged by the condenser lens 28 on the mask 30, causes the reduction lens 32 to reduce the light passing through the pattern of the mask 30, and irradiates the reduced mask pattern on a wafer W to expose the mask pattern.
When a pattern is transferred to a wafer by using a reduction projection exposure device, and when identical patterns in the same exposure field, e.g., wiring patterns each including a large number of identical lines are to be transferred, the wiring pattern transferred to a central region of the exposure field is different from the wiring pattern transferred to a peripheral region of the same exposure field in the line widths of the wiring patterns.
For example, in an independent-line pattern, the line widths of the line-like pattern in the central region and the peripheral region are different from each other, i.e., the line width varies in the exposure field. As important factors which cause the variation, a lens aberration which occurs in a lens of the reduction image-forming optical system and a .sigma.-error which occurs in a fly-eye lens of the illumination optical system are known. In this case, the isolated line indicates a pattern in which another pattern is not present in a region having a width which is four to five times the line width of the isolated line.
The .sigma.-error indicates a variation in coherent of light or non-uniformity of the light intensity of mask illumination. As the .sigma.-error is large, the variation in coherent is large, and the non-uniformity of the light intensity of the mask illumination is great.
Therefore, in order to keep the dimensional uniformity of the line width of a line, the illumination optical system and the reduction image-forming optical system are inevitably adjusted.
Adjustment of the illumination optical system and the reduction image-forming optical system can also be performed by directly measuring the aberration of a lens by using a wave front measurer before the lens is incorporated in the exposure device. However, once the lens is incorporated in the exposure device, detachment of the lens from the exposure device, measurement of the lens aberration, adjustment of the lens aberration, and incorporation of the lens in the exposure device again are very cumbersome. In addition, this operation requires great skill, and is actually difficult to be performed.
Therefore, conventionally, the dimensions of a resist pattern obtained by test exposure are measured to determine whether the dimensional variation is caused by the illumination optical system or the reduction image-forming optical system on the basis of the following indexes.
The first index is a best focus difference between large-dimension and small-dimension L&S patterns. The best focus difference and a spherical aberration have a predetermined relationship, as shown in FIG. 2A. Since the spherical aberration is large when the best focus difference is large, the spherical aberration can be calculated by calculating the best focus difference. The best focus difference is the difference between the focal length of the large pattern of the L&S patterns and the focal length of the small pattern of the L&S patterns.
The second index is a linewidth abnormality. The linewidth abnormality is calculated by equation: EQU linewidth abnormality=(L.sub.1 -L.sub.5)/(L.sub.1 +L.sub.5),
where L.sub.1 and L.sub.5 are the line widths of five L&S patterns at both the ends, respectively. The linewidth abnormality and a coma aberration aberration have a predetermined relationship, as shown in FIG. 2B. Since the coma aberration aberration is large when the linewidth abnormality is large, the coma aberration can be calculated by calculating the linewidth abnormality.
The third index is a line width difference, the line width difference is calculated by equation: EQU line width difference={(L.sub.1 +L.sub.5)/2}-L.sub.3,
where L.sub.1 and L.sub.5 are line widths of five L&S patterns at both the ends, respectively, and L.sub.3 is a line width of the five L&S patterns at the center.
The proximity effect of patterns which are close to each other can be estimated by the size of the line width difference. Since the values of L.sub.1 and L.sub.5 largely vary depending on both the lens aberration of a projection lens and the .sigma.-error, the proximity effect mentioned here is a proximity effect including all the factors.
However, the above method of determining whether adjustment is required or not has various problems.
First, all the indexes described above are related to a lens aberration or a proximity effect varied by the lens aberration and a .sigma.-error, and the size of only the .sigma.-error cannot be detected. In other words, in the conventional methods based on the first to third indexes, whether adjustment of a reduction image-forming optical system is required or not can be determined, but whether adjustment of an illumination optical system is required or not cannot be independently determined.
Second, the relationship between the best focus difference and the spherical aberration is not limited to the relationship shown in FIG. 2A. In general, the relationship shown in FIG. 3A is frequently employed. If the best focus difference is approximate to 0, the actual spherical aberration is often large.
Similarly, the relationship between the linewidth abnormality and the coma aberration is not limited to the relationship shown in FIG. 2B. In general, the relationship shown in FIG. 3B is frequently employed. If the linewidth abnormality is approximate to 0, the actual coma aberration is often large.
The drawbacks described above are caused by the following reasons. That is, the measurement values of a linewidth abnormality and a best focus difference which are actually measured have problems related to precision. For example, the focal distance of a small-dimension L&S pattern is technically very difficult to be measured, and the measurement value of the best focus difference easily includes an error. In addition, the errors of mask pattern dimensions directly adversely affect the measurement value of the linewidth abnormality, and an error easily occurs in the linewidth abnormality.
Third, if a best focus difference and a linewidth abnormality are measured in the form which is free from the measurement error to adjust the exposure device such that the best focus difference and the linewidth abnormality become 0, a practical problem that the dimensional uniformity of an actual pattern is poor is posed because of a problem on the optical design of the exposure device and a manufacturing error.
In this state, determination of whether adjustment of the exposure device is required or not is actually difficult. If an exposure process is performed by using an exposure device which has not been adjusted, a pattern having good dimensional uniformity cannot be transferred.